| Topic: |
Science > Physics |
| User: |
"PD" |
| Date: |
01 Feb 2005 03:33:25 PM |
| Object: |
Re: Help: I need to understand the Lorentz force |
[Posted to a.s.p, s.p, s.p.e. only]
Gordon wrote:
On Thu, 27 Jan 2005 15:45:00 +0000 (UTC), "PD"
<pdraper@yahoo.com> wrote:
[snip]
For a similar shock-and-awe demonstration of the power of cross
products, spin up a bicycle wheel so that it has a sizable angular
momentum. Now hold up *one* end of the axle with your finger and let
go
of the other. Gravity will exert a torque on the wheel, and you can
predict the direction of the torque using the right hand rule. The
direction of the torque will tell you which direction the angular
momentum vector will change. Thus, you will be able to predict not
only
that the wheel will not fall off your finger, but which direction it
will precess. Why is it this way? Because it works.
PD
I think of it in a simpler way (see below).
To get a clear understanding of what's going on here, visualize
holding the above wheel in front of you with the axis oriented
horizontally, left to right and the wheel spinning such that the
lower tangential velocity is toward you, and the top tangential
velocity away from you.
Curling the fingers of your right hand around the axle in the direction
the wheel is turning, your thumb points left. This is the direction of
the angular momentum of the wheel.
Now, if you release the left hand end of the axle, this will
apply a torque that will tend to move the lower edge of the wheel
toward your right, and the upper edge to your right.
That is, if the wheel weren't spinning, it would rotate
counter-clockwise as you are looking at the wheel. You're holding it at
a point right of its center of mass, and gravity acts at the center of
mass, and so gravity is going to exert a torque around the pivot where
you're holding it. Imagine curling the fingers of your right hand (but
don't do it - it's holding the axle up!) around the direction the wheel
would fall once you let go, and your thumb will point directly toward
you. That's the direction of the torque that gravity applies to the
wheel.
Combine the initial tangential velocity for each increment of
mass at the bottom edge of the wheel, with the velocity that will
be imposed upon these same incremental mass elements, due to the
torque from the asymmetrical support of the axle. The resultant
velocity will be toward your right-rear.
In the case of the mass increments at the lower edge of the
wheel, they will proceed on along the path dictated by the
circular shape of the wheel, but they will all have a bit of
newly acquired momentum toward your right. This will then tend to
move the wheel about a vertical axis, such as to move the wheel's
rear edge toward your right.
The torque tells you the rate of change of the angular momentum vector,
which we can write in shorthand as
T = dL/dt
where T is torque, L is angular momentum, t is time, and for giggles
let's suppose d means "change in".
Algebra then tells us that the change in the angular momentum vector is
going to be the torque times the time interval. (We'll take the time
interval to be short.)
dL = T dt.
What this means is that the angular momentum vector has to change in
such a way that the change points in the same direction as the torque.
But if the wheel fell, that would NOT accomplish that purpose. The
angular momentum vector would start out pointing to the left and then
end up pointing downward. That's no good. The torque vector points
toward you. That, then, is the direction the angular momentum vector
has to go.
A similar thought process for the top of the wheel will lead to
the conclusion that the individual mass increments will be given
a velocity to your left, in addition to their initial forward
component of velocity. This resultant velocity from these two
sources will combine with the result that the top-front region of
the wheel to be moved to your left.
Et voila! The wheel does not fall but turns so that the wheel's axis
turns (precesses) in a horizontal plane, exactly as you predicted!
As one more test, slow the wheel down a little and repeat. The torque
due to gravity is the same, so dL is the same, too. But L is smaller
than it was before, so dL has a bigger effect. Thus we predict that it
will precess faster now. And when we try it, we are delighted at the
depth and power of this simple analysis to predict things we would have
never guessed.
PD
This thought process can be applied at any point around the
circumference of the wheel, with the same result.
.
|
|
| User: "Androcles" |
|
| Title: Re: Help: I need to understand the Lorentz force |
01 Feb 2005 03:43:58 PM |
|
|
"PD" <pdraper@yahoo.com> wrote in message
news:1107293605.679380.99450@c13g2000cwb.googlegroups.com...
[Posted to a.s.p, s.p, s.p.e. only]
*****, this is public forum. If you are too ashamed of what you
write,
do it by private email, jerk.
Androcles.
.
|
|
|
| User: "Franz Heymann" |
|
| Title: Re: Help: I need to understand the Lorentz force |
01 Feb 2005 04:01:06 PM |
|
|
"Androcles" <dummy@dummy.com> wrote in message
news:yKSLd.42932$v8.40420@fe2.news.blueyonder.co.uk...
"PD" <pdraper@yahoo.com> wrote in message
news:1107293605.679380.99450@c13g2000cwb.googlegroups.com...
[Posted to a.s.p, s.p, s.p.e. only]
*****, this is public forum. If you are too ashamed of what you
write,
do it by private email, jerk.
PD must have made pulp of you in that protracted argument which you
failed so spectacularly, if the result is that you stalk him with the
crap you uttered in the above sentence.
Franz
.
|
|
|
| User: "Ken S. Tucker" |
|
| Title: Re: Help: I need to understand the Lorentz force |
01 Feb 2005 04:13:11 PM |
|
|
Fun thing about this NG, happy memories...
As a brat, my old boy had an electric motor running
on his bench, nothing attached, I touching the
spinning part and burned my finger. So I turn
it off, and touch it again, and it wasn't hot!
Yes, learning physics threw pain....
Ken
.
|
|
|
| User: "Androcles" |
|
| Title: Re: Help: I need to understand the Lorentz force |
01 Feb 2005 04:39:20 PM |
|
|
"Ken S. Tucker" <dynamics@vianet.on.ca> wrote in message
news:1107295991.663895.41940@o13g2000cwo.googlegroups.com...
Fun thing about this NG, happy memories...
As a brat, my old boy had an electric motor running
on his bench, nothing attached, I touching the
spinning part and burned my finger. So I turn
it off, and touch it again, and it wasn't hot!
Yes, learning physics threw pain....
Ken
My old pop had a heart attack (almost) when he caught me with a bedside
lamp with the bulb socket unscrewed. It was a British plastic (bakelite,
then) type, not the American brass variety, and I was watching the spark
from the built-in switch.
"Look at this, Dad!" 240V, as well, not the American 110V.
I was 11 at the time.
Next came an old cast iron based doorbell and a 6V flashlight battery.
It wasn't working, but there was an adjusting screw on the contacts. I
got a pretty powerful jolt while adjusting it, and the next thing was to
unscrew the bell, attach some wires on each side of the contacts and put
some brass tubes at the other ends.
After some experiments again, I found the optimum setting of the
contacts.
"Hold these" I said to my sister, and switched on.
That cost me a "Got to your room, NOW!"
Next day I took it to school. Oh, what fun...
Androcles.
.
|
|
|
| User: "Franz Heymann" |
|
| Title: Re: Help: I need to understand the Lorentz force |
02 Feb 2005 06:00:21 AM |
|
|
"Androcles" <dummy@dummy.com> wrote in message
news:syTLd.25139$n9.3642@fe3.news.blueyonder.co.uk...
"Ken S. Tucker" <dynamics@vianet.on.ca> wrote in message
news:1107295991.663895.41940@o13g2000cwo.googlegroups.com...
Fun thing about this NG, happy memories...
As a brat, my old boy had an electric motor running
on his bench, nothing attached, I touching the
spinning part and burned my finger. So I turn
it off, and touch it again, and it wasn't hot!
Yes, learning physics threw pain....
Ken
My old pop had a heart attack (almost) when he caught me with a
bedside
lamp with the bulb socket unscrewed. It was a British plastic
(bakelite,
then) type, not the American brass variety, and I was watching the
spark
from the built-in switch.
"Look at this, Dad!" 240V, as well, not the American 110V.
I was 11 at the time.
Next came an old cast iron based doorbell and a 6V flashlight
battery.
It wasn't working, but there was an adjusting screw on the contacts.
I
got a pretty powerful jolt while adjusting it, and the next thing
was to
unscrew the bell, attach some wires on each side of the contacts and
put
some brass tubes at the other ends.
After some experiments again, I found the optimum setting of the
contacts.
"Hold these" I said to my sister, and switched on.
That cost me a "Got to your room, NOW!"
Next day I took it to school. Oh, what fun...
And you don't seem to have progressed much beyond that juvenile stage
yet.
Franz
.
|
|
|
|
| User: "Ken S. Tucker" |
|
| Title: Re: Help: I need to understand the Lorentz force |
01 Feb 2005 05:05:05 PM |
|
|
I was smart enough to be dangerous, but not smart
enough to know how fuckin stupid I was! (maybe I'm
still a brat).
My kind old boy had a virtual junk yard of electrical
stuff in his basement, including many old cords.
In grade school I cut the cord of a plug short, and
twisted the ends together, aka a fuse blowing
machine...cool.
Well, in no time my friends wanted one. Next
thing that happens is the bloody public school
starts winking out, and friends parents start
complaining about blown fuses. After about
a week of havoc, we let it be known we'd
punch out anyone who used one again, that
stopped it.
Ken
.
|
|
|
| User: "Androcles Androcles@ MyPlace.org" |
|
| Title: Re: Help: I need to understand the Lorentz force |
01 Feb 2005 06:41:41 PM |
|
|
"Ken S. Tucker" <dynamics@vianet.on.ca> wrote in message
news:1107299105.610868.238270@o13g2000cwo.googlegroups.com...
I was smart enough to be dangerous, but not smart
enough to know how fuckin stupid I was! (maybe I'm
still a brat).
My kind old boy had a virtual junk yard of electrical
stuff in his basement, including many old cords.
In grade school I cut the cord of a plug short, and
twisted the ends together, aka a fuse blowing
machine...cool.
Well, in no time my friends wanted one. Next
thing that happens is the bloody public school
starts winking out, and friends parents start
complaining about blown fuses. After about
a week of havoc, we let it be known we'd
punch out anyone who used one again, that
stopped it.
Ken
Ah.... I was 10 years old, and my pal Roland Pilcher didn't know that
a coil of wire wound around iron was an electromagnet. I did, you see,
but I'd never tried it, and it always seemed that the wire was pretty
thin
and the chunk of iron was pretty big. Anyway, once I told Roland about
this,
well, what did size matter? Out came the erector set, one skinny axle
with two brass gear wheels to act as cheeks to keep the wire on, the
longest length of flex we could find in his basement. 15 minutes of
winding this on by hand, and me saying "No, more, more!" as Roland said
"Enough?" and we'd filled the spool. Now, I was still apprehensive, it
just didn't fell right, somehow. But Roland had total faith in me, I
could climb trees without fear, and he had to be brave too. So into the
socket went the wires, no plug on them. Bang! Roland's dad wasn't too
pleased, he had to go into the basement in the dark to find the fuse
box. That ended that experiment. Good thing the fuse blew, though. That
coil would have gotten mighty hot.
Androcles.
.
|
|
|
| User: "Ken S. Tucker" |
|
| Title: Re: Help: I need to understand the Lorentz force |
01 Feb 2005 07:13:02 PM |
|
|
Yup...fuses/circuit breakers are vital if brats are around...
The old boy had piles of Nichrome (stove wire) stashed,
so I did an experiment, on my bedroom floor (oak) and
kept shortening the NiCr wire connection to the 120V.
Normally in a stove it's red, possibly with an orange
tinge. Well, you connect the wires out of the socket
and make the connection shorter and shorter on the
NiCr length, is really super...goes from orange to
yellow to white.
But get this, I observed it goes slightly green just
before it melts down. I presume that was a Quantum
experience.
Ken
.
|
|
|
| User: "Androcles Androcles@ MyPlace.org" |
|
| Title: Re: Help: I need to understand the Lorentz force |
01 Feb 2005 08:14:12 PM |
|
|
"Ken S. Tucker" <dynamics@vianet.on.ca> wrote in message
news:1107306782.467519.204700@l41g2000cwc.googlegroups.com...
Yup...fuses/circuit breakers are vital if brats are around...
Yeah, but we weren't being brats. We used to ring doorbells
and run away to be brats. Playing with electricity was.. well...
honest, nothing to be ashamed of.
Unless you shocked your sister deliberately, of course.
Parents should know the difference. Then I had two girls
myself and they never did anything like it, so I couldn't
put that into practice. Still, my grandson knows how to take out
the potting shed window with a good kick to a football, he's
scored 3 times now. How much fun is that?
The old boy had piles of Nichrome (stove wire) stashed,
so I did an experiment, on my bedroom floor (oak) and
kept shortening the NiCr wire connection to the 120V.
Normally in a stove it's red, possibly with an orange
tinge. Well, you connect the wires out of the socket
and make the connection shorter and shorter on the
NiCr length, is really super...goes from orange to
yellow to white.
But get this, I observed it goes slightly green just
before it melts down. I presume that was a Quantum
experience.
Oxidation, perhaps? The nearest I came to that was a deliberately
broken light bulb with the filament left intact to light a firecracker.
Androcles
Ken
.
|
|
|
| User: "Ken S. Tucker" |
|
| Title: Re: Help: I need to understand the Lorentz force |
02 Feb 2005 03:37:42 PM |
|
|
Androcles wrote:
"Ken S. Tucker" <dynamics@vianet.on.ca> wrote in message
news:1107306782.467519.204700@l41g2000cwc.googlegroups.com...
Yup...fuses/circuit breakers are vital if brats are around...
Yeah, but we weren't being brats. We used to ring doorbells
and run away to be brats. Playing with electricity was.. well...
honest, nothing to be ashamed of.
Unless you shocked your sister deliberately, of course.
Parents should know the difference. Then I had two girls
myself and they never did anything like it, so I couldn't
put that into practice. Still, my grandson knows how to take out
the potting shed window with a good kick to a football, he's
scored 3 times now. How much fun is that?
The old boy had piles of Nichrome (stove wire) stashed,
so I did an experiment, on my bedroom floor (oak) and
kept shortening the NiCr wire connection to the 120V.
Normally in a stove it's red, possibly with an orange
tinge. Well, you connect the wires out of the socket
and make the connection shorter and shorter on the
NiCr length, is really super...goes from orange to
yellow to white.
But get this, I observed it goes slightly green just
before it melts down. I presume that was a Quantum
experience.
Oxidation, perhaps?
That might be it, duplicate the experiment,
maybe in nitrogen, that's cheap.
The nearest I came to that was a deliberately
broken light bulb with the filament left intact to light a
firecracker.
Androcles
A good friend of mine is a world famous rocketeer,
"Richard Nakka", he has an excellent web-site.
I designed "ultra-low current" igniters to assist
rocketry ignition, based on filaments, he has my
designs posted.
BTW, I was a brat when I started building those,
but to my credit, it made setting off pipe-bombs
a lot safer, compared to a fuse, likely prevented
kids from getting injured as easily.
Ken
.
|
|
|
|
|
| User: "Timo Nieminen" |
|
| Title: Re: Help: I need to understand the Lorentz force |
01 Feb 2005 07:52:09 PM |
|
|
On Wed, 1 Feb 2005, Ken S. Tucker wrote:
The old boy had piles of Nichrome (stove wire) stashed,
so I did an experiment, on my bedroom floor (oak) and
kept shortening the NiCr wire connection to the 120V.
Normally in a stove it's red, possibly with an orange
tinge. Well, you connect the wires out of the socket
and make the connection shorter and shorter on the
NiCr length, is really super...goes from orange to
yellow to white.
But get this, I observed it goes slightly green just
before it melts down. I presume that was a Quantum
experience.
I used to get a similar effect when shorting out old clapped-out tractor
batteries with fencing wire (though I don't recall it getting to white
hot) - the same green effect just before it melts. I assumed it was a
chemical thing, some impurities being cooked out just as the wire starts
to melt. Well, the green could be emission from the vapour, most certainly
a Quantum experience.
--
Timo
.
|
|
|
| User: "Ken S. Tucker" |
|
| Title: Re: Help: I need to understand the Lorentz force |
01 Feb 2005 08:23:24 PM |
|
|
Timo Nieminen wrote:
On Wed, 1 Feb 2005, Ken S. Tucker wrote:
The old boy had piles of Nichrome (stove wire) stashed,
so I did an experiment, on my bedroom floor (oak) and
kept shortening the NiCr wire connection to the 120V.
Normally in a stove it's red, possibly with an orange
tinge. Well, you connect the wires out of the socket
and make the connection shorter and shorter on the
NiCr length, is really super...goes from orange to
yellow to white.
But get this, I observed it goes slightly green just
before it melts down. I presume that was a Quantum
experience.
I used to get a similar effect when shorting out old clapped-out
tractor
batteries with fencing wire (though I don't recall it getting to
white
hot) - the same green effect just before it melts. I assumed it was a
chemical thing, some impurities being cooked out just as the wire
starts
to melt. Well, the green could be emission from the vapour, most
certainly
a Quantum experience.
--
Timo
I moved the wires providing the current and voltage
slowly to shorten the length of the NiCr wire, I
*presume* the chemical oxidization element
of that compound (NiCr) was eliminated, but you might
be right. There was no gas emmision, until the wire
melted at a point, then a bit of smoke, likely oxide
formation at that point.
It's a cheap and easy experiment to reproduce,
go ahead, it's fun.
Thanks Timo
Ken
.
|
|
|
|
|
|
|
|
|
|
|
|
|
Related Articles |
|
|
